1. Field of the Invention
The present invention relates generally to a multi-hop relay Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for providing a synchronous relay service in a multi-hop relay BWA communication system.
2. Description of the Related Art
In fourth-Generation (4G) communication systems, radii of cells are reduced to achieve a higher transmission rate and accommodate a greater number of calls. Centralized network design with conventional technology is not viable for the 4G mobile communication systems. Thus, the 4G mobile communication systems should allow for distributed control and implementations and actively adjust to environment changes, such as addition of a new Base Station (BS). In order to accomplish this, the 4G mobile communication systems should use a self-configurable wireless network.
For realtime deployment of a self-configurable wireless network, technologies used for an ad-hoc network are introduced to the 4G mobile communication systems. A major example is a multi-hop relay network configured by introducing a multi-hop relay scheme used for the ad-hoc network to a cellular network using fixed BSs.
Since communications are performed between a BS and a Mobile Station (MS) via a direct link, a highly reliable radio communication link can be easily established between the BS and the MS in the cellular network. However, the fixedness of BSs impedes flexible wireless network configuration, which makes it difficult to provide efficient services in a radio environment that experiences a fluctuating traffic distribution and a great change in the number of calls.
To avert this problem, a relay scheme is adopted in which data is conveyed through multiple hops via neighbor MSs or neighbor Relay Stations (RSs). A multi-hop relay scheme facilitates fast network reconfiguration adaptive to an environmental change and renders an overall wireless network operation efficient. Also, the multi-hop relay scheme can provide a better-quality radio channel to an MS by installing an RS between the BS and the MS and thus establishing a multi-hop relay path via the RS. In addition, the multi-hop relay scheme can expand cell coverage because it can provide high-speed data channels to MSs in a cell boundary area where channel conditions from the BS are poor.
FIG. 1 is a diagram illustrating the configuration of a wireless communication system using a typical relay scheme.
Referring to FIG. 1, an MS 110 (MS1) within a service area (i.e., coverage area) 101 of a BS 100 communicates with the BS 100 via a direct link. An MS 120 (MS2), which is located outside the service area 101 of the BS 100 and thus is in poor channel condition, communicates with the BS 100 via a relay link of an RS 130.
Using the RS 130, the BS 100 can communicate with MSs that are in poor channel condition because they are located outside the service area 101 of the BS 100 or in a shadowing area experiencing severe shielding effects of buildings.
FIG. 2 is a diagram illustrating a frame structure for a wireless communication system using a conventional relay scheme.
Referring to FIG. 2, the frame is divided into a DownLink (DL) subframe 200 and an UpLink (UL) subframe 230.
The DL subframe 200 is divided into a first section 210 for providing a service from a BS via a direct link and a second section 220 for providing a service from an RS via a relay link.
For the first section 210, the BS configures a BS DL subframe that will be transmitted to the RS or an MS connected via a direct link The BS DL subframe a sync channel (preamble) 211, a control channel 213, and a DL burst 215.
For the second section 220, the RS configures an RS DL subframe that will be transmitted to a subordinate RS or an MS connected via a relay link. The RS DL subframe an RS sync channel (RS preamble) 221, a control channel 223, and a DL burst 225.
The UL subframe 230 is divided into a first section 231 for communication with a BS via a direct link and a second section 233 for communication with an RS via a relay link.
For the first section 231, a BS UL subframe is configured to transmit control information and traffic to the BS from the RS or an MS connected through a direct link to the BS. For the second section 233, an RS UL subframe is configured to transmit control information and traffic to the RS from an MS connected via a relay link.
A guard interval of a Transmit/receive Transition Gap (TTG) 240 exists between the DL subframe 200 and the UL subframe 230. Also, a guard interval of a Receive/transmit Transition Gap (RTG) 250 exists between the DL subframe 200 and the UL subframe of the previous frame.
When communications are performed using the frames illustrated in FIG. 2 MSs have different frame timings depending on the service-providing entities (e.g., a BS or an RS). For example, for the downlink, an MS receiving a service from the BS receives a service through the BS DL subframe of the first section 210. For the uplink, an MS receiving a service from the RS receives a service through the RS DL subframe of the second section 220.
As described above, when the MSs receiving services from the BS or the RS operate asynchronously, the synchronization and handover of each MS are difficult to implement.